1. Field of the Invention
The present invention relates to a lead-free piezo-electric porcelain composition for use as a piezo-electric device or the like, a piezo-electric device using the same, and a method for producing the same.
2. Description of the Related Art
Many of hitherto mass-produced piezo-electric porcelains (piezo-electric ceramics) have been composed of a PZT-based (lead titanate zirconate-based) material, and contain lead. However, in recent years, in order to eliminate the adverse influence of lead on the environment, there has been a desire to develop a lead-free piezo-electric porcelain. As a material of such a lead-free piezo-electric porcelain (referred to as a “lead-free piezo-electric porcelain composition”), the use of a perovskite oxide containing K, Na and Nb (or tantalum) as main metal components (hereinafter referred to as a “KNN phase”) has been proposed. However, the KNN phase itself has a problem in that it is inferior in sinterability and humidity resistance.
In addressing such problems, JP-A-2001-190146 (Patent Document 1) proposes to improve piezo-electric characteristics by allowing glass to exist at the grain boundary of crystal grains of a compound represented by the general formula KxNa(1-x)NbO3.
Moreover, Japanese Patent No. 5214373 (Patent Document 2) proposes to improve piezo-electric characteristics and the like by using [K1-xNax]1-yLiy[Nb1-z-wTazSbw]O3 as a main phase and incorporating K3Nb3O6Si2O7 as a sub-phase to micronize the main phase.
WO 2013/008418 (Patent Document 3) of the present applicant discloses improving piezo-electric characteristics and the like by using the KNN phase as a first crystal phase (main phase), incorporating an NTK phase containing Nb, Ti and K as main metal components of a second crystal phase (sub-phase), and filling the spaces between the crystal particles of the first crystal phase with the second crystal phase.
However, since the crystal structure of the KNN phase is nearly cubic, a large number of voids are inevitably generated with grain growth. In Patent Document 1, the grain boundary of the KIND phase can be filled with glass, but there is a problem in that the piezo-electric characteristics cannot be sufficiently improved by filling the grain boundary. Moreover, even in Patent Document 2, the main phase can be micronized by incorporating K3Nb3O6Si2O7 as a sub-phase. However, since a large number of voids remain, there is also a problem in that the piezo-electric characteristics cannot be sufficiently improved. In Patent Document 3, the piezo-electric characteristics and the like are improved by filling the spaces between the crystal particles of the first crystal phase with the NTK phase. However, the piezo-electric constant d33 is at most about 250 pC/N, and there is a problem in that sufficient characteristics are not necessarily obtained. Accordingly, in order to place various devices using a lead-free piezo-electric porcelain composition into practical use, there has been a desire to develop a lead-free piezo-electric porcelain composition having enhanced piezo-electric characteristics.